Here's a bunch of quick answers....
I am wondering where/ when the schematic might be posted
The schematic is likely sometime next week. No schematic has been drawn yet, so it's going to take a little while.
I found out about the 4.0 launch this morning (after some level of following beta testing since it started and much anticipation ever since it was first announced) and I ordered immediately. My order has shipped the same day.
We managed to ship almost all the orders placed in the first ~4 hours after the announcement.
Robin & Angel are testing and packaging more T4s right now and we're going to package up as many of today's orders as we can this evening. You probably won't be seeing a lot of writing from me for a little while....
the blurb says it has two USB high speed ports, does it support a USB host port like the 3.6 ?
Yes, it supports USB host with the same USBHost_t36 library, but the hardware support is not exactly like Teensy 3.6.
On 3.6 you get a USB current limit chip and 5 through-hole locations to solder a connector. On Teensy 4.0 the 2nd USB port signals just come to pads on the bottom of the board. It's a much more DIY approach. For the beta test, we had boards with pogo pins to touch those pads, and that board added a TPS2055A current limit switch. You might manage to get by without a current limit chip, but the experience isn't much fun because hot-plugging a USB device can draw too much current and possibly crash (brown-out reset) your Teensy.
I just noticed the updated back side graphic, i guess there is no more external AREF? is there a way to get the same sort of performance from the ADCs with potentiometers across a 3 or 3.3v supply?
Indeed this chip doesn't support AREF at all. The ADC always uses the 3.3V power as its reference.
So yes, you can read pots connected between 3.3V and GND.
But also no, not quite. The ADC in this new chip is much faster, but its resolution isn't as good as the ones on Teensy 3.x. The effective number of bits is about 10. NXP calls it 12 bits, but then they also call the one in the Kinetis chips 16 bits.
you need the latest Rev C of the audio adaptor (has 100 ohm resistor on MCLK). not all of audio lib is working ...
EDIT: actually,audio adaptor won't work (see post #16 below). it worked with special breakout boards that Paul provided...
Yup. We're planning a Rev D version later this month, which will be the exact same hardware but with the signals routed to the correct pins for Teensy 4.0.
In the beta, we actually used Rev B audio boards. The beta test board connected them and had a 100 ohm resistor on the MCLK pin. We recently updated the audio shield to Rev C, which just added that 100 ohm resistor, so if you wire it up to a Teensy 4.0 on a breadboard or any other way where you connect the signals to the right places, it will "just work".
If you have a Rev B audio shield, you'll need to add the wires and add a 100 ohm resistor in series with MCLK.
I only see up to Teensy 3.6 available for board selection in the Arduino IDE (ver 1.8.9). Will the Teensy 4.0 board be supported soon?
Yes, some some definition of "soon". Likely Friday morning.
Does the Teensy 4.0 accept 5.0V on ADC pins or just 3.3V?
No, absolutely not. Except the VIN and VUSB pins, nothing is 5V tolerant on these new parts.